The present invention relates to a four wheel steering system for improving cornering performances of a vehicle.
Conventional four wheel steering systems are disclosed in a plurality of publications. Examples are Japanese patent provisional publication no. 60-85062, and U.S. Pat. Nos. 4,588,039; 4,586,581; and 4,579,186. Steering systems of these examples are designed to steer rear wheels in response to steering operation of front wheels on the premise that all of the front and rear wheels are equipped with tires of the same characteristic.
In general, a tire exhibits a characteristic of a cornering force CF versus a tire slip angle, as shown in FIG. 9. A cornering power CP indicated in FIG. 9 is a rate of rise of the cornering force with respect to increase in the tire slip angle, usually evaluated at or near zero slip angle. The cornering force increases with increase of the tire slip angle, and reaches a maximum value at a maximum cornering force generating slip angle .alpha..sub.1. From the nature of a tire, the maximum cornering force generating slip angle .alpha..sub.1 is decreased when the cornering power CP is increased, as shown in by a solid line in FIG. 9, and the cornering power Cp is decreased when the maximum cornering force generating slip angle is increased, as shown by a one dot chain line in FIG. 9.
A vehicle equipped with tires having a high cornering power is advantageous in that the front wheels can improve the steering response and the rear wheels can improve cornering performances in a cornering operation in which a vehicle lateral acceleration or the tire slip angle remains low. However, in a cornering operation producing a high lateral acceleration or a great tire slip angle, the ability of grip of the tire readily becomes insufficient especially in the rear wheels because of the maximum cornering force generating slip angle being small, so that the vehicle easily falls into spin.
On the other hand, a vehicle equipped with tires of a low cornering power is advantageous in the cornering operation accompanied by a high lateral acceleration or a great tire slip angle. Because the maximum cornering force generating tire slip angle of such tires is high, the possibility of shortage of the grip in the rear wheels is reduced. Therefore, the vehicle having tires of a low cornering power can prevent spin-out, and widen a permissible range of the yaw angle. However, in the state wherein the lateral acceleration or the slip angle is at a low level, the insufficiency of the cornering power impairs the steering response in the front wheels, and the cornering performances in the rear wheels.
Therefore, it is desirable to provide the front wheels with a relatively high cornering power, and to provide the rear wheels with a relatively high cornering power during a cornering operation at low lateral acceleration or small tire slip angle, and with a relatively low cornering power during a cornering operation at high lateral acceleration or great tire slip angle.
However, the conventional steering systems are not arranged to adjust the tire characteristics in accordance with a cornering condition of the vehicle, and the tire characteristic is appropriate only in a limited operating condition.
Especially when all the wheels are equipped with tires having a great cornering power to meet the requirements on the front wheels, the rear wheel tires readily become insufficient in ability of grip during increase of the tire slip angle, and increases the possibility of spin of the vehicle. In this case, the cornering force drops rapidly after the cornering force is increased to a maximum by increase of the slip angle. Therefore, the vehicle changes its behavior so rapidly at an initial stage of spin, that it is difficult to control the direction of the vehicle by steering.